[SCEV] Include type when printing constant max backedge taken count

When printing the result of the analysis, i8 -1 and i64 -1 are quite
different in terms of analysis quality.  In a recent conversion with
a new contributor, we ran into exactly this confusion.

Adding the type for constant scevs more globally seems worthwhile, but
introduces a much larger test diff.  I'm splitting this off first since
it addresses the immediate need, and then going to do some further
changes to clarify a few related bits of analysis result output.